1. Trends in Precipitation and Stream Discharge over the Past Century for the Continental United States Andrew Simon 1 and Lauren Klimetz 1,2 1 USDA-ARS National Sedimentation Laboratory, Oxford, MS 2 Dept. Of Civil and Environmental Engineering, U. of Tennessee, Knoxville, TN andrew.simon@ars.usda.gov National Sedimentation Laboratory

2. Background and Context The National Sedimentation Laboratory has been conducting research in support of U.S. EPA to determine “background” or “reference” rates of suspended-sediment transport in the continental United States. “Reference” defined as a “stable” (not static) channel where sediment delivered from upstream can be transported through the reach without the channel aggrading, degrading widening or narrowing over a period of years; Because of differences across the country in climate, topography, geology and land use, a regional approach was required; Level III Ecoregions were used.

3. Level III Ecoregions and Available Data

4. Example: EPA Region 8

5. Example “Reference” Yield

6. However… Questions raised about how current flow and transport conditions may be different from historical, sampling periods; In Oklahoma, a mean-annual precipitation difference between wet and dry periods of 33% led to a 100% difference in runoff, which in turn led to a 183% difference in sediment yield. (Garbrecht, 2008); Results not surprising as more sediment moves at higher discharges; The question becomes do wet periods cause transport ratings to shift ?... AND/OR Do they de-stabilize channel systems?

7. Transport Ratings Stable Ratings Unstable Ratings

8. Precipitation Data U.S. Historical Climatology Network (USHCN) operated by the National Climatic Data Center (NCDC) of NOAA; http://www.ncdc.noaa.gov/oa/climate/research/ushcn/ushcn.html Source: Details: More than 1,000 station nationwide Monthly data to nearest 0.01” Does not differentiate between rainfall and snowfall

9. Discharge Data U.S. Geological Survey gauging station records http://nwis.waterdata.usgs.gov/nwis/dv Source: Details: More than 6,000 stations nationwide Used more than 2,900 (with minimum 30 suspended-sediment samples and associated instantaneous discharge) Mean-daily data to three significant figures

10. Summary of Data

11. Precipitation (mm) All but ER 42 increasing Dust Bowl Era Wet period in 1990’s

12. Water Yield (m 3 /s/km 2 ) Shows combined effects of changes in precipitation and anthropogenic influences All Ecoregions except ER 46* and ER 48 show less discharge per unit area Indication of significant human impacts

13. Annual Departure from Long- Term Mean Water Yield (%) Trends for all Ecoregions show decrease, except ER 46* and 48 Most secondary peaks around 1970 Less discharge per unit area

14. Water Retention: Dam Construction Summary Reached peak retention per unit area Farm Bill led to significant upstream retention 1970-1980

15. Potential Human Impacts construction of thousands of dams agriculture logging surface mining coal-bed methane production channelization irrigation, tile drainage

16. Decadal Trends (Wet/Dry Periods) Precipitation Water Yield (m 3 /s/km 2 ) By 1970-1980, Ecoregions 17, 42 and 43 have the most dams

17. Water Yield Per Unit of Precipitation

18. Summary of Changes Over Past 100 Years Note: * denotes that data period started during the 1920’s or 1930’s (Dust Bowl Era), resulting in higher than expected increases. * (in mm)(in percent) (m 3 /s/km 2 ) (in percent) 15Northern Rockies85.915.7-0.00026-1.63-26.4 17Middle Rockies 4.01 1.04-0.01066-68.9-81.8 42Northwestern Glaciated Plains-7.74-2.04-0.00045-20.6-84.9 43Northwestern Great Plains24.76.35-0.00131-79.1-74.4 46Northern Glaciated Plains11.1 2.090.000823043*1070* 48Lake Agassiz Plain45.58.940.002613530 Change in precipitation over 100 year period Change in water yield over 100 year period Level III Ecoregion Change in water yield per unit precipitation (in percent)

19. National Context Winter: December – February Spring: March – May Summer: June – August Autumn: September - November Seasonal Breakdowns:

20. Seasonal Changes in Precipitation: 100 Years Spring Summer Autumn Winter

21. Change in Precipitation (Percent)

22. Change in Precipitation (mm)

23. Seasonal Changes in Water Yield: 100 Years Spring Summer Autumn Winter

24. Change in Water Yield (Percent)

25. Change in Water Yield Per Unit Precipitation Indicative of Human Impacts

26. Median Suspended-Sediment Concentrations

27. Median Suspended-Sediment Yields

28. Summary and Conclusions EPA 8: Although increased precipitation, significant reductions in water yield in the four western ecoregions due to human impacts; well below average since the 1970s-80s when most dam construction in the region was completed. EPA 8: Changes in precipitation combined with human-induced changes have caused dramatic shifts in discharge rates such that historically diverse ecoregions now provide similar amounts of water (per unit drainage area) to streams (ie. Middle Rockies similar to the Northwestern Great Plains). National Context Most of the U.S. experiencing more precipitation than 100 years ago. Dramatic shifts in timing: More in autumn, much less in winter nationwide. Although numerous regions are experiencing significantly more precipitation, discharge per unit area has been drastically reduced in many areas, particularly west of the Mississippi River. The drastic reductions in water yield can be largely attributed to human impacts given the general increase in precipitation in these regions. This probably has significant effects on geomorphic and ecological processes, and habitat.